There has been known a technique for detecting, in an image of a document read by an image reader, an image corresponding to, for example, a hole (e.g., a perforation or a punched hole) formed in the document using a hole punch and eliminating the detected image from the image of the document.
For example, a known image reader obtains an image density of a reference image by reading a white reference member and obtains an image density of a document image by reading a document. The image density of the reference image is higher than the image density of a blank portion (having a color of white) of the document image. In a case that the document has a punched hole, an image density of an outline of the punched hole is lower than the image density of the blank portion. The image reader detects a circle having a lower image density than the image density of the blank portion in the document image. Thereafter, the image reader determines whether an image density of an image enclosed by the detected circle is equal to the image density of the reference image. When the image density of the image enclosed by the circle is equal to the image density of the reference image, the image reader determines that the document has a punched hole.